Such positioning devices are also referred to as gantry drive or X-Y gantry, and are used in many fields of technology. The processing of planar substrates, such as wafers or PC boards, is a typical application for such gantry drives. Due to advances in miniaturization, an ever better positioning accuracy is demanded here, as well.
European Patent No. 2 066 996 describes a positioning device in gantry type of construction, in which, in order to determine position more accurately, position-measuring systems are used which, in addition to the position in the actual measuring direction along a linear guide, are also able to ascertain small deviations of the position in a direction transversely thereto (for example, guiding errors). For that purpose, scales are used which, in addition to the actual measuring track in the measuring direction, also bear what is referred to a straightness track, on which such small deviations transversely to the measuring direction are able to be read. Such scales are also referred to as 1Dplus scales, because, in addition to covering one measuring direction (1D), they also permit a measurement in a further direction, albeit only small deviations in this direction. Such scales and corresponding position-measuring systems are described in greater detail in the publication HEIDENHAIN INFO, Electronics, Volume 1, 2009. A use of such scales on a positioning device in gantry type of construction is described therein, as well.
For the actual measuring direction, a typical 1Dplus scale bears an incremental track or, more generally, a measuring track, which may include fine, short lines transversely to the measuring direction that, for example, are scanned optically by a scanning head movable relative to the scale. The straightness track includes a few long lines, which are arranged parallel to the actual measuring direction (and therefore transversely to the direction of the guiding errors) next to the incremental track. This straightness track is also scanned by the scanning head. By scanning the scale tracks, periodic signals are obtained in response to a relative movement between scale and scanning head. By counting the periods and subdividing the individual periods (interpolation), the extent of the displacement and thus a position may therefore be determined. Thus, position values in two directions independent of each other are able to be read from a 1Dplus scale.
Conventional dual-gantry systems include two displaceable cross members arranged between the two linear guides in the first direction. PCT International Published Patent Application No. WO 02/067648 describes a machine tool for processing printed circuit boards, which has a second cross member having additional processing tools in order to increase productivity.
Among end uses for such positioning devices in gantry type of construction, are those in which a substrate placed on a movable table must be moved under a stationary tool. In that context, one moving direction is executed step-by-step (stepping), the other moving direction is executed with a continuous movement of the substrate (scanning). The continuous scanning movement is performed along the cross member of the positioning device, and the step-by-step stepping movement is performed along the two parallel linear axes. For many practical applications (e.g., wafer inspection with the aid of bright-field microscopy), a straight-line scanning movement with an allowed deviation of 20 nanometers at most from an ideal straight line is required.
However, such accuracies are unlikely to be achieved using conventional positioning devices in gantry type of construction. The heat input of the drive in the cross member and the forces necessary to move the wafer table cause a deformation and even vibrations of the structures of the positioning device, which make a precise position determination and therefore also a precise position control impossible.